1. Field of the Invention
The present invention relates to an endoscope apparatus using an image pickup device which is inserted in a body cavity.
2. Description of the Prior Art
A typical conventional endoscope apparatus used a camera attached to the eyepiece of a fiberscope having an image guide comprising optical fibers, thereby picking up the images of a body cavity. At that time, ambiguous images were often created due to movement of the body cavity, etc., which used to be solved by increasing the shuttering rate of the camera. However, a TV monitor is normally used as a display means with an endoscope system using the solid-state image pickup device. Therefore, a minimum shuttering rate of the camera is 1/30 sec to 1/60 sec (these values correspond to the period of the TV monitor per frame or field).
In addition, said time period of 1/30 sec to 1/60 sec should be synchronized with the frame frequency of the TV monitor or field frequency.
Such shuttering operation cannot be practically realized because a shutter is mechanically driven.
Consequently, the foregoing image pickup requires such a series of operations that first the TV monitor is set to blanking state then the shutter is opened to display only the image of a required frame or field, then again set to blanking state while the shutter is then returned to close.
Such operations not only are complicated but also require that the iris be opened wide according to the luminance of the TV monitor and film sensitivity. Therefore, focal depth becomes shallower, making it very difficult to reduce out-focusing for the image displayed on the TV monitor.
Another method is also proposed and used to pick up images, in which the image information for a frame or field is stored in the memory, sent to and displayed on the TV monitor while displayed image is being picked up.
At this time, a problem is how long time is required to collect image information to be stored in the memory. Referring to FIG. 1 (a) in which light is emitted for each field, for example, time for collecting image information per frame becomes (1/60+T) sec where t.sub.VD is the pulse width of its synchronous pulse VD and T is the light emitting time interval of a light source for picking up images.
Referring to FIG. 1 (b) where light is emitted at the end of a field and the beginning of the next field for collecting images, on the other hand, time for collecting image information per frame becomes (t.sub.VD +2T) sec.
In either case, time for emitting light is T sec per field. However, time required for structuring image for a frame with the case of FIG. 1 (b) is much shorter.
Consequently, the case of FIG. 1 (b) brings out the image with less out-focusing.
Another problem is long or short time for emitting light as described above. With this emitting time T shorter, the foregoing time required for forming a frame can be reduced further. However, with this time reduced in excess, ratio S/N deteriorates because of the performance characteristics of the solid-state image pickup element.